Thoracic Muscles

Thoracic muscles are a group of muscles located within the thoracic wall and thoracic cavity, playing essential roles in respiration, movement of the upper limbs, and stabilization of the chest wall. These muscles include intrinsic muscles (confined to the thoracic wall) and extrinsic muscles (that span beyond the thorax but influence thoracic function). They are organized into superficial, intermediate, and deep layers based on their anatomical relationships and function.

Location and Classification

The thoracic muscles are primarily situated between the ribs, around the thoracic cavity, and on the anterior and posterior surfaces of the thorax. They can be classified into the following groups:

• Extrinsic muscles of the thorax: Originate or insert outside the thorax but affect thoracic movement—includes pectoralis major, pectoralis minor, serratus anterior, and subclavius.
• Intrinsic muscles of the thorax: Located entirely within the thoracic wall—includes intercostal muscles (external, internal, innermost), subcostal muscles, and transversus thoracis.
• Diaphragm: A large, dome-shaped muscle that separates the thoracic and abdominal cavities and is the principal muscle of respiration.

Superficial Thoracic Muscles

Pectoralis Major

• Origin: Clavicle, sternum, and costal cartilages of ribs 1–6
• Insertion: Lateral lip of the intertubercular groove of the humerus
• Function: Adducts and medially rotates the arm; helps with inspiration when arms are fixed
• Innervation: Medial and lateral pectoral nerves (C5–T1)

Pectoralis Minor

• Origin: Anterior surfaces of ribs 3–5
• Insertion: Coracoid process of the scapula
• Function: Stabilizes scapula and assists in elevating ribs during deep inspiration
• Innervation: Medial pectoral nerve (C8–T1)

Serratus Anterior

• Origin: Lateral parts of ribs 1–8
• Insertion: Medial border of the scapula
• Function: Protracts scapula and assists with upward rotation; helps with forced respiration
• Innervation: Long thoracic nerve (C5–C7)

Subclavius

• Origin: First rib and its costal cartilage
• Insertion: Inferior surface of the middle third of the clavicle
• Function: Stabilizes and depresses the clavicle
• Innervation: Nerve to subclavius (C5–C6)

Intrinsic Muscles of the Thoracic Wall

External Intercostal Muscles

• Origin: Inferior border of one rib
• Insertion: Superior border of the rib below
• Fiber direction: Obliquely downward and forward
• Function: Elevate ribs during inspiration
• Innervation: Intercostal nerves (T1–T11)

Internal Intercostal Muscles

• Origin: Inferior border of one rib
• Insertion: Superior border of the rib below
• Fiber direction: Obliquely downward and backward (perpendicular to external intercostals)
• Function: Depress ribs during forced expiration
• Innervation: Intercostal nerves (T1–T11)

Innermost Intercostal Muscles

• Location: Deepest layer of intercostals; separated from internal intercostals by intercostal nerves and vessels
• Function: Assist internal intercostals in rib depression

Subcostal Muscles

• Location: Posterior thoracic wall; span multiple ribs
• Function: Weak rib depressors; may stabilize ribs

Transversus Thoracis

• Origin: Posterior surface of the lower sternum and xiphoid process
• Insertion: Internal surfaces of costal cartilages 2–6
• Function: Weak rib depressor; may help anchor intercostal spaces

Diaphragm

• Shape: Dome-shaped muscular partition between thorax and abdomen
• Origin: Sternal part (xiphoid), costal part (ribs 7–12), and lumbar part (L1–L3 vertebral bodies via crura)
• Insertion: Central tendon of the diaphragm
• Function: Main muscle of inspiration; contracts to increase thoracic volume
• Innervation: Phrenic nerve (C3–C5)

Functions of Thoracic Muscles

• Enable respiratory movements by elevating or depressing the ribs
• Stabilize the thoracic wall during upper limb and trunk activity
• Contribute to forced breathing during physical exertion
• Assist in posture and support of the pectoral girdle

Layering and Relationships

From superficial to deep:

1. Pectoralis major and minor, serratus anterior, subclavius (extrinsic)
2. External intercostals (superficial intrinsic layer)
3. Internal intercostals (middle layer)
4. Innermost intercostals and transversus thoracis (deepest layer)

Neurovascular bundles (intercostal vein, artery, nerve) lie between the internal and innermost intercostal layers, within the costal groove of each rib.

Blood Supply

• Arterial: Internal thoracic artery and its branches (anterior intercostal arteries), posterior intercostal arteries (from the thoracic aorta)
• Venous: Drainage via internal thoracic, azygos, and hemiazygos systems

Innervation

• Intercostal muscles: Intercostal nerves (T1–T11)
• Diaphragm: Phrenic nerve (C3–C5)
• Pectoral muscles: Medial and lateral pectoral nerves
• Serratus anterior: Long thoracic nerve

Clinical Relevance

• Intercostal neuralgia: Irritation of intercostal nerves may cause pain along the thoracic wall
• Diaphragmatic paralysis: Caused by phrenic nerve damage; impairs respiration
• Long thoracic nerve injury: May result in winged scapula due to serratus anterior dysfunction